This invention relates to sailboats, and, more particularly, to the keels used thereon.
A sailboat includes a hull that sits in the water, a mast extending upwardly from the hull, sails supported by the mast, and either a centerboard or fixed keel extending downwardly from the hull into the water. The sails catch the wind and cause the hull to move forwardly through the water. A sailboat cannot sail directly into the wind, but can sail in a generally windward direction. With skill and a combination of maneuvers, the sailor can move a sailboat in any desired direction.
Because of the design of the sails, a sailboat can sail to windward, in a direction no less than about 15 to 25 degrees from the wind, depending upon the design of the boat and the skill of the sailor. Headway directly upwind is achieved in a series of sequential maneuvers called tacks, in which the boat is first sailed windward with the wind over one side of the bow, and then turned through the wind so that the wind comes over the other side of the bow. In each tack, some headway upwind is achieved even though the boat does not move directly into the wind, and eventually the sailboat reaches an upwind objective after sailing a zig-zag course covering a distance greater than the straight line distance from the initial position to the upwind objective.
When a sailboat sails to windward, the forces on the sails can be resolved into a thrust component that moves the sailboat forwardly through the water and a drift component that pushes the sailboat sideways in a downwind direction. The sailboat therefore moves in a net direction that is forward, but also is slight downwind opposite to the net intended direction of movement. The sideways drift is called leeway.
The downwardly projecting centerboard or keel of the boat offers resistance to the leeway produced by the sideways sail force, but at least some leeway remains. This leeway is being constantly accumulated, as there is a downwind movement as long as the sailboat is being sailed into the wind. The leeway significantly increases the time required for the sailboat to sail from its downwind starting position to the upwind objective, as it forces the sailboat to sail much further to make up for the accumulated sideways movement.
There have been numerous attempts to reduce the amount of leeway. As previously mentioned, a movable centerboard or fixed keel extending into the water below the sailboat presents a broad surface to resist sideways drift. There have also been attempts to modify the shape of the centerboard or keel to provide a lifting force to counteract the sideways drift. These attempts have been based upon the observation that the centerboard or keel moving through the water is somewhat similar to the wing of an airplane that creates a lift as the wing is moved through the air. This lift of an airplane wing causes the airplane to move upward against the force of gravity, and the corresponding lift of a sailboat centerboard or keel that extends downwardly can cause the sailboat to be lifted in the upwind direction, thereby countering the sideways drift producing the leeway. The use of such a centerboard or keel requires that it be adjustable during operation, as on the average, left or port lift is required half the time when the boat is sailed to windward, and on the average right or starboard lift is required half the time. The use of an underwater wing, which has recently become popular for racing sailboats, has an entirely different purpose, although it operates by a similar principle, for the lift of such a wing is largely upwardly (rather than laterally, as for a leeway reducing keel) to reduce the wetted area of the hull and thence its drag as it moves forwardly.
In one approach, a lifting centerboard is formed by bending a flexible centerboard so as to be shaped like an airplane wing. In another approach, a lifting centerboard assembly is formed by providing two movable centerboards, each having an oppositely disposed airfoil shape. The appropriate centerboard is lowered into the water to obtain lift in one direction, and is withdrawn and replaced with the other centerboard to obtain lift in the other direction. The creation of the proper lift is relatively easily attained with the smaller sailboats that use movable centerboards, as the centerboards can be readily adjusted to achieve either port or starboard lift in varying degrees as needed.
Application of the principles of lifting bodies to fixed keels is conceptually similar, but is technically much more complex and difficult, for several reasons. Fixed keels are typically used in larger sailboats. The keels are usually filled with lead or other dense material to act as ballast for the sailboat. For example, the keels of 12-meter sailboats may extend 10 feet below the surface of the water, and weigh 40,000 to 50,000 pounds, The keels cannot be readily warped or lifted partially out of the water, as can movable centerboards, due to their size and weight. The fixed keels are structurally strong, and thence resistant to deflection. The fixed keels must also remain in the water during the entire course of use, and therefore must be resistant to fouling by marine organisms and seaweed.
There have been proposed approaches to using fixed keels as selectively adjustable lifting bodies. Such approaches have involved complex mechanical systems of arms and levers that extend downwardly from the hull into the keel. The mechanisms are used to selectively deflect the sides of the keel outwardly and inwardly as needed, so that the upwind side of the keel can be bowed to produce a lifting body.
Such mechanical systems are not generally practical for either racing or crusing sailboats. The complex mechanisms are prone to failure, particularly when in contact with corrosive seawater, and failure of any portion of the mechanism results in the keel shape being frozen in its position when the failure occurred. If the failure occurred when the keel was shaped to produce sideways lift, that position is retained until the mechanism is repaired. For a sailboat engaged in a race at the time of failure, the adverse effect of the retained shape on the opposite tack is almost certain to make the sailboat completely noncompetitive. Thus, failure of any part of the mechanism can cause the sailboat to lose the race. Since the most failure-prone components of the mechanism are inside the keel and not accessible during the race, the use of such prior mechanisms is highly risky. The mechanisms are also heavy, and adversely shift the center of gravity of the sailboat.
Accordingly, existing designs for lifting fixed keels have not been widely accepted, and no types of racing or pleasure sailboats are known to use such designs. However, the advantages of using adjustable lifting keels are significant, and development of a technically acceptable lifting keel would undoubtedly result in its widespread use. There therefore exists a need for such a fixed keel which is capable of providing a lifting force to counteract leeway, and is sufficiently reliable to be acceptable for general and racing use. The present invention fulfills this need, and further provides related advantages.